In the US, almost 400,000 patients annually receive balloon angioplasty to improve arterial blood flow. About 40 percent of these procedures are unsuccessful because of restenosis, which is estimated to cost the US health care system approximately 0.8-2 billion dollars annually. Endo-vascular irradiation is a new and innovative approach offering the potential of effective restenosis prevention. Almost all the used/proposed radionuclides have short radioactive half- lives so that, during the useful life of the source, exposure times must be continuously extended because of the decreasing source activity. Since the exposure times are comparable to the half-time of sublethal damage repair (T1/2), the biological effect of a given dose will vary significantly over the practical lifetime of the source. The goal is to produce a reliable user-friendly software tool allowing the physician to find the optimal way to adjust for a given reduction in the activity (and therefore the dose rate) of the source, so that the biological effect will remain unchanged. The basic information required is the T1/2 of the target cells, together with the other relevant radiobiological response parameter (alpha/beta). In Phase I, these will be determined for one coronary artery smooth muscle cell line of human origin, and a pilot computer program will be written and assessed, to adjust exposure times to obtain a constant biologically effective dose as the radioactive source decays and treatment times vary. PROPOSED COMMERCIAL APPLICATION Restenosis costs the US health care system approximately 0.8-2 billion dollars/yr. Vascular irradiation is a new technique potentially offering effective restenosis prevention, and is likely to have widespread application. Because the radioactive sources used decay over their useful lifetime, a flexible tool giving biologically-based adjustments in the dose prescription for changes in dose rate will be essential.